Flexible Wall Permeameters for Research on Microplastic Transport in Soil: Validity, Advantages, and Constraints

Microplastics (MPs) are emerging contaminants detected in a range of ecosystems, including soils. Rigid wall columns are widely used to study MP transport in soil. However, they are limited in their ability to control boundary conditions and prevent sidewall leakage in studying low-permeability soils. Flexible wall permeameters (FWPs), commonly used for measuring soil permeability, offer more precise control of confining pressure and leakage, making them a promising alternative that remains under-explored. This study evaluates FWPs for MP transport through mass-balance experiments to assess the extent to which the artificial retention of MP by FWP may interfere with testing and reduce its validity. Tests are conducted under different FWP components, pressure gradients, ionic strengths of solution, and MP sizes and concentrations. Results show baseline mass recoveries greater than 80% but declining for larger MPs (5 μm) and high ionic strengths. Agglomeration and filtration by filter paper were the primary causes of retention. Overall, FWPs proved effective for studying MP transport for particles of ≤1 μm at ionic strengths smaller than 0.03 M, with near-complete recovery achieved when avoiding filter papers. In addition, MP flow-through experiments conducted on sandy and bentonite clay soil samples confirmed FWP suitability as a soil column. Finally, guidance for its effective utilization for MP soil transport studies was provided.